Additive for improved thermal bonding between dissimilar polymeric layers

ABSTRACT

The invention relates to a method for thermally bonding dissimilar polymeric layers on a paper or paperboard substrate whereby the bonding strength between the dissimilar polymeric layers is substantially increased. The method utilizes a resin of the structureA-B-A&#39;wherein each of A and A&#39; is selected from a hydrocarbonous material which is melt compatible with one of the polymeric layers and B is a hydrocarbonous material having characteristics compatible with the other polymeric layer in order to modify the dissimilar polymeric materials in such a manner that there is an increased thermal bonding strength between the dissimilar polymeric materials.

This application is a division of application Ser. No. 08/445,908 filedMay 22, 1995, now U.S. Pat. No. 5,679,201 issued Oct. 21, 1997, and is acontinuation-in-part of application Ser. No. 08/279,631 filed Jul. 22,1994, now U.S. Pat. No. 5,464,691 issued Nov. 7, 1995.

BACKGROUND

The present invention relates to polymeric coated composites forpackaging and, in particular, to methods for improving the thermalbonding between dissimilar polymeric coatings.

Polyolefinic materials such as low density polyethylene (LDPE),polypropylene (PP), polybutylene (PB) and polystyrene (PS) are used inhigh volumes by the packaging industry as one component of a laminatecomposite film for coating substrates such as paper, metal foils,nonwoven fabrics and the like. Such polyolefinic materials havesubstantially non-polar characteristics and thus are not typically wellsuited for thermal adhesion to polymeric materials having more polarcharacteristics.

Methods for making polymeric coated paper and paperboard substrates foruse as containers and cartons are well known. See for example, U.S. Pat.Nos. 4,698,246; 4,701,360; 4,789,575; 4,806,399; 4,888,222; and5,002,833.

In many circumstances, it is desirable to use a non-polar polymericmaterial on one surface of a substrate for a carton or container and amore polar or otherwise dissimilar polymeric material on anothersurface. During the process of container construction, the finishedpackage often contains an overlap joint. The overlap joint may be bondedwith an adhesive, however it is preferable in many cases to thermallyseal the joint by mating the two surfaces under heat and pressure sothat they are, in effect, fused together. Thermally bonded surfacesoffer the potential of providing greater or more uniform bondingstrength than other bonding techniques so that fewer joint failuresoccur. Difficulties often arise, however, when attempting to obtain astrong bond between dissimilar polymeric materials by the use of thermalbonding techniques alone. Adhesives, on the other hand, may not besuitable in many situations, particularly in food or medicalapplications where contamination of the food or medical materials withthe adhesive may occur.

It is therefore an object of the invention to provide an improvedpolymer coated substrate for packaging.

Another object of the invention is to provide a method for improving thethermal bonding between dissimilar polymeric materials.

A further object of the invention is provide a method for thermallybonding dissimilar polymeric materials on a coated substrate.

A still further object of the invention is to increase the thermalbonding strength between dissimilar polymeric materials without the needto use a separate or discrete polymeric tie layer between the dissimilarpolymeric materials.

Another object of the invention is to improve the thermal bondingstrength between dissimilar polymeric materials without the need forelaborate or complicated coating techniques.

THE INVENTION

With regard to the foregoing and other objects, the invention provides amethod for thermally bonding dissimilar polymeric layers on a paper orpaperboard substrate. The method comprises applying to a first surfaceof the substrate a first layer of polymeric material. A second layer ofa dissimilar polymeric material is applied to a second surface of thesubstrate. Either the first or second layer, or both of them, includesfrom about 0.5 wt. % to about 10 wt. % of a triblock resin additive ofthe structure

    A--B--A'

wherein each of A and A' is selected from a hydrocarbonous materialhaving polymeric characteristics similar to one of the first or secondpolymeric layers and B is a hydrocarbonous material having polymericcharacteristics similar to the other polymeric layer. The first andsecond coated surfaces of the substrate are then thermally bondedtogether whereby the bonding strength between the first and secondlayers is substantially greater than the bonding strength of the samepolymeric materials thermally bonded together in the absence of theresin.

As used herein in connection with the polymeric layers the terms,"similar" and "dissimilar" refer principally to the immiscibility of thematerials. For example, one polymeric material which exhibits anappreciably different polarity from another polymeric material wouldgenerally exhibit appreciable immiscibility in the other polymericmaterial and, therefore, would not be likely to establish an acceptablystrong thermal bond when the two materials are placed in contact andheated to a temperature sufficient to promote thermal bonding. As to thedegree of dissimilarity of any two polymeric materials to which thepresent invention may be applied, those of ordinary skill willappreciate that this is dependent upon whether any two polymericmaterials exhibit a sufficiently strong thermal bond in a particularsituation based on specific criteria. In those cases where thedissimilar polymeric materials do not exhibit acceptable thermal bondingstrength to meet the criteria or requirements, the invention would beexpected to find useful application for materially improving thestrength of the bond.

The terminology "thermal bonding" and variants thereof as used hereinrefers to a heating or other application of energy to the respectivelayers in interfacial contact so as to attempt to induce an infusion,migration or intermingling of mass between the polymeric layers at theinterface. For example, sufficient heat energy may be applied to causeone or both of the materials to flow and therefore promote mass transferof the materials one into the other, and this transfer is often assistedby application of pressure. The process may occur as in a "wetting" ofthe surface of one material which remains essentially solid while theother has been rendered flowable by heat energy. This infusion,migration or intermingling is promoted to a significant degree by theuse of the triblock additive of the invention which promotes a lockingtogether of the layers without the need for a separate tie layer.

According to another aspect of the invention, substrate surfaces coatedwith dissimilar polymeric materials may now be thermally joined orbonded together without the need for use of relatively expensiveadhesives or high molecular weight polymeric tie layers. Furthermore,the thermally bonded surfaces have a strength which is sufficient tosignificantly reduce the number and frequency of joint failures inpolymeric coated paperboard products. These and other advantages of theinvention may be obtained by either coating or admixing the resindisclosed herein with one of the polymeric coatings, preferably admixingthe resin with the polymeric coating which is to be surface wet by theother polymeric coating, then thermally bonding the two coated surfaces,one to the other. In many cases, the resin is admixed with the highermelting polymeric coating, as this coating will typically be wet by thelower melting polymeric coating.

In one of its embodiments, the invention provides a method for formingan improved heat sealable coated substrate. The method comprisesadmixing a first polymeric material with from about 0.05 to about 10% byweight of an amphiphilic resin of the structure

    A--B--A'

wherein each of A and A' is an essentially non-polar hydrocarbonousgroup and B is an essentially polar group derived from a telechelicdiol. The mixture of amphiphilic resin and first polymeric material isapplied to a first surface of a substrate. A second surface of thesubstrate is coated with a second polymeric material which is dissimilarto the first polymeric material. Finally, the first and second coatedsurfaces are thermally bonded together whereby the bonding strengthbetween the first and second coated surfaces is substantially greaterthan the bonding strength of the same coated surfaces bonded under thesame condition but without the amphiphilic resin.

In yet another embodiment, the invention provides a method for thermallybonding a first surface of a substrate coated with a substantiallynon-polar polymeric material to a second surface of the substrate coatedwith a polar polymeric material. The method comprises increasing thebonding activity of the surface of the second polymeric material by useof a resin of the structure

    A--B--A'

wherein each of A and A' is selected from an essentially non-polarhydrocarbonous group and B is an essentially polar hydrocarbonous groupwhereby the bonding strength between the first and second coatedsurfaces of the substrate is substantially greater the bonding strengthof the same coated surfaces in the absence of the resin.

Again, the dissimilar polymeric materials are materials which, bynature, do not thermally bond to each other according to the criteriarequired for a particular application. In general terms, the dissimilarpolymeric materials may be described as polar or non-polar materials,however, this terminology is used in a relative, rather than in anabsolute sense. The dissimilar polymeric materials may also be describedin terms of their relative surface energies. One polymeric material mayhave a surface energy of greater than about 40 dynes/cm² and the othermaterial may have a surface energy of less than about 40 dynes/cm². Thedissimilar polymeric materials may also be described as materials whichoppose surface wetting one to the other under conditions wherein one orboth of the materials is in molten form. Regardless of the terminologyused to characterize the dissimilar polymeric materials, the presentinvention provides a means for improving a thermal bond betweensubstrates coated with such dissimilar polymeric materials.

One class of polymeric material to which the invention may apply,referred to generally as non-polar polymeric materials, includespolyolefinic materials selected from polyethylene (PE), polypropylene(PP), alpha-olefin modified polyethylene and polypropylene, polybutylene(PB), polystyrene (PS), poly(4-methyl 1-pentene), or mixtures of two ormore of the foregoing. A particularly useful application of theinvention is in respect to PE or PP, and in particular, low densitypolyethylene (LDPE).

The amount of substantially non-polar polymeric material applied to onesurface of the substrate may range from about 5 to about 15 pounds per3000 square feet of substrate. The coating may be a single coating ofnon-polar polymeric material, it may be a plurality of individualcoatings totalling the about 5 to about 15 pounds per 3000 square feetof substrate, or it may be one of a plurality of polymeric coatings andadhesives or tie layers provided the non-polar polymeric material is theexposed surface coating on the substrate.

A polymeric layer composed of a thermoplastic material having a polaritysubstantially greater than the non-polar material is applied to anopposing surface of the paper or paperboard substrate that is to bebonded to the non-polar coated surface. As noted herein, the absolutepolarities of the so called "non-polar" and "more polar" polymericlayers are not critical to the invention, provided there is a sufficientdifference in polarity to adversely affect the thermal bondingcharacteristics between the dissimilar polymers in the absence of theresin described herein.

As with the first coating, the amount of more polar coating applied tothe opposing surface of the substrate may range from about 5 to about 15pounds per 3000 square feet of substrate and may be applied as a singlecoating, as a plurality of coatings, or as one of a plurality ofpolymeric coatings and adhesives or tie layers provided it is theexposed surface coating on the surface of the substrate.

The more polar polymeric material is typically a polymeric materialcontaining polar substituents or containing groups having electronwithdrawing or electron donating characteristics. Accordingly, the morepolar polymeric material may include polyamides such as nylon,polyesters such as polyethylene terephthalate (PET), polybutyleneterephthalate (PBT) and glycol modified polyethylene terephthalate(PETG), polyvinyl- and vinylidene halides such as polyvinylidenechloride and polyvinyl chloride, polycarbonate (PC) and polyolefinicalcohols such as poly(ethylene-co-vinyl alcohol) (EVOH). Particularlypreferred coatings for food packaging applications are PET and EVOH. Awidely used EVOH polymeric material is sold under the trade name EVAL EPby the Kuraray Co. Ltd. of Osaka, Japan and a widely used PET polymericmaterial is KODAPAK 12440 PET which is available from Eastman ChemicalProducts Inc., of Kingsport, Tenn. Among the available PC polymericmaterials is LEXAN 104-111 resin which is available from The GeneralElectric Company of Schenectady, N.Y.

Methods for coating the substrate with the polymeric materials are wellknown. Such methods include brushing the layers on the substrate as wellas roll, rod, doctor blade, spray coating or extrusion coating methods.

Prior to, essentially simultaneously with or subsequent to coating thesubstrate surfaces with the dissimilar polymeric materials anamphiphilic resin of the structure

    A--B--A'

wherein each of A and A' is selected from an essentially non-polarhydrocarbonous group and B is an essentially polar group derived from atelechelic diol, is incorporated within at least one of the coatings.Thus, the amphiphilic resin may be admixed and co-applied with at leastone of the polymeric layers or it may be applied in a separate coatingstep to one or both of the polymeric layers after the polymeric layershave been applied to the substrate.

For application of the amphiphilic resin to the one or both of thepolymeric coated surfaces, the amphiphilic resin may first be dissolvedin a solvent such as tetrahydrofuran (THF). The particular solvent isnot believed to be critical to the invention. Accordingly, any solventwhich forms a liquid dispersion or single phase solution or mixture withthe amphiphilic resin may be used. Suitable solvents are those havingsolvating power for both blocks of the copolymer and having a relativelyhigh volatility such as acetone, chloroform, ethylacetate, methyl ethylketone, and the like.

The amount of amphiphilic resin in the solvent may range from about 0.5to about 10% by weight, with about 1% by weight being preferred for mostcoating methods. The solution concentration may readily be adjusted toprovide a sub-micrometer thickness of amphiphilic resin on one or bothof the polymeric coated surfaces.

In an alternative embodiment, the amphiphilic resin is admixed with thepolymeric material and co-applied onto the substrate. Preferably, theamphiphilic resin is admixed with the higher melting polymeric materialso that when the material cools, a portion of the resin migrates to thesurface of the polymeric material thereby modifying its surfaceproperties to the extent that the bonding activity of the polymericmaterial is substantially increased. Under thermal bonding conditions,the lower melting polymeric material will interact with the A or Bblocks of the resin thereby wetting and bonding to an increased degreewith the surface of the higher melting material.

The amount of amphiphilic resin admixed with the polymeric material mayvary depending on the particular amphiphilic resin and the polymericmaterial's molecular weight and crystallinity. For most applications, asuitable amount of amphiphilic resin is in the range of from about 0.5to about 5% by weight, most preferably from about 1 to about 2% byweight.

Methods for producing the admixture of polymeric material andamphiphilic resin are well known. Accordingly, the amphiphilic resin maybe admixed with the polymeric material by melt blending the twocomponents, blending two solutions containing the polymeric material andamphiphilic resin, blending the two components in a high shear mixer, oradding the amphiphilic resin as a solid or liquid to the polymericmaterial during application to the substrate. Melt blending of theamphiphilic resin and higher melting polymeric material may beaccomplished, for example, in a thermostatted mixer or compoundingextruder. Alternatively, the amphiphilic resin may be added to thepolymeric material during work-up immediately after polymerization. Theorder in which the two components is mixed is not believed to becritical to the invention. Accordingly, the amphiphilic resin may beadded to the polymeric material or the polymeric material may be addedto the amphiphilic resin.

A preferred amphiphilic resin is a compound of the formula ##STR1##wherein each of R and R¹ is selected from the group consisting of alkyl,aryl, alkylaryl groups and, acyl and arylacyl derivatives of analiphatic or aliphatic/aromatic mono-acid with a molecular weight offrom about 200 to about 500 daltons each of D and D¹ is selected from O,--NR³ --, S and CO₂ ; each of R² and R³ is selected from the groupconsisting of H, CH₃ and C₂ H₅ ; each of n and p is and integer from 0to 8, provided when p=0, n is greater than 0; each of m and r is aninteger selected from 0 to 20, provided when r=0, m is greater than 0;and s is an integer from 1 to 20.

Examples of alkyl, aryl, alkylaryl groups and, acyl and arylacylderivatives of an aliphatic or aliphatic/aromatic mono-acid withmolecular weights of from about 200 to about 500 daltons include, butare not limited to, alkylbenzenes, aliphatic alcohols, acyl derivativesof saturated fatty acids having carbon atom chain lengths of from about10 to 26 atoms, soya and tall oil fatty acids, alkylbenzoic acids andtall oil, wood and gum rosin acids, and the like.

In a more preferred amphiphilic resin, R and R¹ are the same and aremost preferably lipophilic rosin; R² is hydrogen; D and D' are CO₂ ; nis 5; and m is 1; r is 0; and s is an integer from 1 to 20. In anotherpreferred amphiphilic resin, R and R¹ are the same and are mostpreferably lipophilic rosin; R² is hydrogen; D and D' are CO₂ ; m, n andr are 1; p is 3; and s is an integer from 1 to 20, most preferably 9. Inyet another preferred amphiphilic resin, R and R¹ are the same and aremost preferably lipophilic rosin; R² is hydrogen; D and D' are CO₂ ; m,n, p and r are 1; and s is an integer from 1 to 20. Particularlypreferred amphiphilic resins include a rosin/polycaprolactone (PCL)triblock, a rosin/poly(ethylene adipate) (PEA) triblock and arosin/poly(ethylene succinate) (PES) triblock.

The amphiphilic resin may be formed by the reaction of polyglycols,polyimines, polyesters or polysulfides with hydrophobic compounds suchas fatty acids, rosin acids, alkylphenols or aryl or aliphatic alcohols.The chain length of the hydrophilic segment, polyethylene glycol, forexample, may be from 2 to 20 units (where a unit is composed of 1, 2, 3or 4 carbon atoms and one polar group, i.e. an oxygen, nitrogen orsulfur atoms or carboxyl group) with a preferred length of about 9units. The hydrophilic or lipophilic groups may have chain lengths offrom about 10 to about 26 carbon atoms. The preferred aromatic,aliphatic or mixed alcohols generally have molecular weights from about200 to about 500 daltons.

The increase in thermal bonding strength between the first and secondcoated surfaces may be determined by comparing the surface activity ofunmodified polymeric layers, such as polyethylene terephthalate, againstthe activity of the admixture of amphiphilic resin and polymericmaterial, such as obtained from applying the admixture of polyethyleneterephthalate and amphiphilic resin described herein to a substrate. Thethermal bonding strength may also be determined by measuring the T-peelstrength required to separate the first coated surface from the secondcoated surface. In the T-peel measurement, a layer of polyolefinicmaterial is applied to the first surface of a substrate such aspaperboard. The more polar polymeric material is applied to a secondsurface of the substrate. Next, a coating of amphiphilic resin isapplied to one or both of the first and second coated surfaces. Prior tothermally bonding the two surfaces, the amphiphilic coating may beenergized by flame treatment, infra-red heating, forced hot air heating,laser heating or the like. The energization of the amphiphilic coatingshould be sufficient to partially melt the polymeric surface coated withthe amphiphilic resin and provide interdiffusion of the amphiphilicresin into the polymeric material.

Heat and pressure are then applied to the physically interfaced firstand second coated substrates to form a bond between the two coatedsurfaces. The amount of force (measured in lb/in.) needed to peel thesecond coated surface away from the first coated surface at an angle of90° is called the T-peel strength.

While the foregoing description generally relates to the formation ofcoated substrates containing a single coating of each polymeric materialon the first and second surfaces, the coated substrate may contain anynumber of sub-coatings provided it contains at least one externalcoating comprising a non-polar polyolefinic material and at least oneexternal coating composed of a more polar material than the firstcoating with an amphiphilic resin available or applied at the physicalinterface between the coatings to enhance the thermal bondingcharacteristics between the two coated surfaces.

Any reference herein to coating or otherwise applying the non-polarand/or more polar coatings on a first or second surface of the substratewill be understood to include applying the coatings to underlyingcoatings which themselves are applied to the substrate or to one or moreother coatings one of which is applied to the substrate. Also, it iswithin the scope of the invention that the first and second surfaces ofthe substrate may be on both or only one side of the substrate, such asin different sections on one side. For example, the first surface may bepart of one surface of the sheet and the second surface may be anotherpart of the same surface of the sheet, with these surfaces beingdiscrete areas or patterns on the sheet in terms of the coatings whichare coplanar but physically separate. When the sheet is folded orassembled into a final carton as in liquid food packing, these areas maybe brought into contact for thermal bonding.

In order to provide a further understanding of the present invention thefollowing examples are given to illustrate, but not to limit theinvention. Examples 1-3 illustrate methods for preparing variousamphiphilic resins which may be used to thermally bond dissimilarpolymeric materials. The general procedure for preparing variousamphiphilic resins is given in U.S. Pat. No. 5,272,196 to Gardiner,incorporated herein by reference as if fully set forth.

EXAMPLE 1

An amphiphilic resin was prepared by the esterification of tall oilrosin (320 grams, 1.1 mols) with poly(caprolactone) diol (415 grams, 0.5mols) (TONE 201, Union Carbide, molecular weight of 830). Two equivalentweight of the poly(caprolactone) diol were reacted with the tall oilrosin, in the presence of 2.7 grams (40 mmols) hypophosphorous acidcatalyst at 260° C. for 20 hours. The reaction was carried out under anitrogen blanket with stirring and an exit condenser to condense andremove water formed during the esterification step. The resultingamphiphilic resin had an acid number of 20 mg KOH/g of product and aGardner color of 5 (molten color). The amphiphilic resin was a viscousliquid produced in 95% yield having the general structure:

    Rosin--{(CH.sub.2).sub.5 CO.sub.2 }.sub.n --Rosin

EXAMPLE 2

An amphiphilic resin was prepared by the esterification of tall oilrosin (320 grams, 1.1 mols) with poly(ethylene adipate)diol (500 grams,0.5 mols) (DESMOPHEN 2500, Bayer Corporation, molecular weight of 1000).Two equivalent weight of the poly(ethylene adipate)diol were reactedwith the tall oil rosin in the presence of 2.7 grams (40 mmols)hypophosphorous acid catalyst at 60° C. for 20 hours. The reaction wascarried out under a nitrogen blanket with stirring and an exit condenserto condense and remove water formed during the esterification step. Theresulting amphiphilic resin had an acid number of 18 mg KOH/g of productand a Gardner color of 5 (molten color). The amphiphilic resin was aviscous liquid produced in 95% yield having the genreral structure:

    Rosin--{(CH.sub.2).sub.2 CO.sub.2 --(CH.sub.2).sub.4 CO.sub.2 }.sub.n --Rosin

EXAMPLE 3

An amphiphilic resin was prepared by the esterification of tall oilfatty acid (TOFA) (282 grams) (Acintol EPG tall Oil fatty acid, ArizonaChemical Company) with poly(caprolactone)diol (415 grams, 0.5 mols)(TONE 20, Union Carbide, molecular weight of 830). Two equivalent weightof the poly(caprolactone)diol were reacted with the tall oil fatty acid,in the presence of 2.7 grams (40 mmols) phosphoric acid catalyst at 220°C. for 20 hours. The reaction was carried out under a nitrogen blanketwith stirring and an exit condenser to condense and remove water formedduring the esterification step. The resulting amphiphilic resin had anacid number of 12 mg KOH/g of product and a Gardner color of 2. Theamphiphilic resin was a viscous liquid produced in 95% yield having thegeneral structure:

    TOFA --{(CH.sub.2).sub.5 CO.sub.2 }.sub.n --TOFA.

Examples 4 and 5 illustrate the increase in thermal bonding strengthbetween dissimilar polymeric coated substrates containing theamphiphilic resin of the invention.

EXAMPLE 4

To demonstrate the increased thermal bonding strength between first andsecond coated surfaces of a paperboard substrate, International PaperJuice Carton board was used as a test sample. The board was extrusioncoated with PE on one surface of the paperboard and PET on the oppositesurface of the paperboard. A 1% by weight solution of an amphiphilicresin similar to the resin of Examples 1 and 2 in tetrahydrofuran (THF)was coated onto the surface of the PET. The solution was allowed to dry1 hour at about 23° C. to insure removal of all of the THF solvent. Nextthe amphiphilic resin coated surface was flame treated simultaneouslywith the PE coated surface by passing the coated board sample rapidlythrough a natural gas flame (Fisher Burner), seven to eight times. Theflame-treated surfaces were mated and pressure applied with asoft-rubber roller (Shore A hardness of 60 to 80) against a glass plate.The pressure applied was sufficient to intimately mate the surfaceswithout compressing the substrate.

The strength of the thermal bond was tested by separating the two piecesat a separation rate of 12 inches (30.5 cm) per minute at an angle of90° (T-peel) according to ASTM D1876-93). The strength improvement wastypified by both an increase in the maximum peel value and an increasein total energy to failure as compared to a control which was preparedin exactly the same manner but without the resin additive. Moreimportantly, the substrate failed before the bond between the surfacesfailed. The results are given in Table 1.

                  TABLE 1                                                         ______________________________________                                                               Total                                                            Maximum      Energy                                                           Peel         at Break Failure                                       Sample    (lb/in.)     (lb/in.) Mode                                          ______________________________________                                        Uncoated  1.92         1.09     interfacial.sup.1                             control                                                                       Coated    2.16         1.44     fiber tear.sup.2                              with 1                                                                        wt. % PCL.sup.3                                                               solution                                                                      Coated    2.67         1.79     fiber tear                                    with 1                                                                        wt. % PEA.sup.4                                                               solution                                                                      ______________________________________                                         .sup.1 interfacial = failure between two pieces of laminate                   .sup.2 fiber tear = failure in laminate structure within paperboard,          usually at PET/paper interface                                                .sup.3 PCL = rosin/polycaprolactone triblock resin                            .sup.4 PEA = rosin/poly(ethylene adipate) triblock resin                 

EXAMPLE 5

The increased thermal bonding strength between PE and EVOH first andsecond coated surfaces of a paperboard substrate was demonstrated usingInternational Paper Juice Carton board as a test sample. The board wasextrusion coated with PE on one surface of the paperboard and EVOH onthe opposite surface of the paperboard. A 1% by weight solution of anamphiphilic resin similar to the resin of Examples 1 and 2 intetrahydrofuran (THF) was coated onto the surface containing the EVOH.The solution was allowed to dry 1 hour at about 23° C. to insure removalof all of the THF solvent. Next the amphiphilic resin coated surface wasflame treated simultaneously with the PE surface by passing the coatedboard sample rapidly through a natural gas flame (Fisher Burner), sevento eight times. The flame-treated surfaces were mated and pressureapplied with a soft-rubber roller (Shore A hardness of 60 to 80) againsta glass plate. The pressure applied was sufficient to intimately matethe surfaces without compressing the substrate.

The strength of the thermal bond was tested by separating at aseparation rate of 12 inches (30.5 cm) per minute at an angle of 90°(T-peel) according to ASTM D1876-93). The strength improvement wastypified by both an increase in the maximum peel value and an increasein total energy to failure as compared to a control prepared in the samemanner but without the resin additive. The results are given in Table 2.

                  TABLE 2                                                         ______________________________________                                                               Total                                                            Maximum      Energy                                                           Peel         at Break Failure                                       Sample    (lb/in.)     (lb/in.) Mode                                          ______________________________________                                        Uncoated  0.47         0.06     interfacial.sup.1                             control                                                                       Coated    1.05         0.12     interfacial                                   with 1                                                                        wt. % PCL.sup.2                                                               solution                                                                      Coated    1.68         0.14     interfacial                                   with 1                                                                        wt. % PEA.sup.3                                                               solution                                                                      ______________________________________                                         .sup.1 interfacial = failure between two pieces of laminate                   .sup.2 PCL = rosin/polycaprolactone triblock resin                            .sup.3 PEA = rosin/poly(ethylene adipate) triblock resin                 

From Tables 1 and 2, it can be seen that the T-peel strength between thePE and PEA/PET coated substrates was about 40% greater than the PE/PETcoated control sample and the strength between the PE and PEA/EVOHcoated substrates was 257% greater than the PE/EVOH coated controlsample. Likewise, the T-peel strength between the PE and PCL coatedsubstrates was about 12.5% greater than the PE/PET coated control sampleand the strength between the PE and PCL/EVOH coated substrates was 123%greater than the PE/EVOH coated control sample. It is believed that theincrease in bond strength may lead to greater package integrity, reducedfailure rates, and allow the seam to contribute structural strength inload bearing applications.

Since topical application of the triblock resin may not be practical ordesirable in all manufacturing processes, the surface energy change ofan admixture of PET and the PCL or PEA triblock resin was demonstrated.An increase in the water contact angle indicates a decrease in thesurface energy of the coating. The results are given in Table 3.

                  TABLE 3                                                         ______________________________________                                        Sample          Water Contact Angle (°)                                ______________________________________                                        PET without triblock                                                                          59                                                            resin                                                                         PET containing 1 wt. %                                                                        65                                                            PCL.sup.1                                                                     PET containing 1 wt. %                                                                        62                                                            PEA.sup.2                                                                     ______________________________________                                         .sup.1 PCL = rosin/polycaprolactone triblock resin                            .sup.2 PEA = rosin/poly(ethelene adipate) triblock resin                 

Having described various and preferred embodiments of the invention andthe benefits and advantages thereof, it will be recognized by those ofordinary skill that variations of the specifically disclosed embodimentsmay be made and, indeed, other or improved embodiments may be madewithin the spirit and scope of the appended claims.

What is claimed is:
 1. A paperboard container which comprises a firstpaperboard substrate area containing a first polymeric material layer, asecond paperboard substrate area containing a second polymeric materiallayer which is dissimilar to the first polymeric material layer, and athermally bonded joint between the first polymeric material layer andthe second polymeric material layer, thereby connecting said first andsecond paperboard substrate areas together, said first polymericmaterial layer containing or having coated thereon from about 0.5 toabout 10 wt. % of a tri-block resin of the structure

    A--B--A

formed by the reaction of an A-block material selected from the groupconsisting of fatty acids, rosin acids, alkyl phenols, and aryl andaliphatic alcohols with a B-block material selected from the groupconsisting of polyglycols, polyimines, polyesters, and polysulfides. 2.The container of claim 1 wherein the A-block and B-block materialsreacted to form the A--B--A tri-block resin comprise a rosin acid and apolyethylene glycol, respectively.
 3. The container of claim 1 whereinthe A-block and B-block materials reacted to form the A--B--A tri-blockresin comprise a rosin acid and polyethylene adipate diol, respectively.4. The container of claim 1 wherein the A-block and B-block materialsreacted to form the A--B--A tri-block resin comprise a rosin acid andpolyethylene succinate diol, respectively.
 5. The container of claim 1wherein the A-block and B-block materials reacted to form the A--B--Atri-block resin comprise a fatty acid a B-block material selected fromthe group consisting of a polycaprolactone diol, a polyethylene adipatediol, and a polyethylene succinate diol, respectively.
 6. A paperboardcontainer which comprises a first paperboard substrate area containing afirst polymeric material layer, a second paperboard substrate areacontaining a second polymeric material layer which is dissimilar to thefirst polymeric material layer, and a thermally bonded joint between thefirst polymeric material layer and the second polymeric material layer,thereby connecting said first and second paperboard substrate areastogether, said first polymeric material layer containing or havingcoated thereon from about 0.5 to about 10 wt. % of a tri-block resin ofthe structure

    A--B--A

wherein the A--B--A tri-block resin is selected from the groupconsisting of rosin/polycaprolactone, rosin/poly(ethylene adipate), androsin/poly(ethylene succinate) tri-blocks.
 7. A paperboard containerwhich comprises a first paperboard substrate area containing a firstpolymeric material layer, a second paperboard substrate area containinga second polymeric material layer which is dissimilar to the firstpolymeric material layer, and a thermally bonded joint between the firstpolymeric material layer and the second polymeric material layer,thereby connecting said first and second paperboard substrate areastogether, said first polymeric material layer containing or havingcoated thereon from about 0.5 to about 10 wt. % of a tri-block resin ofthe structure

    A--B--A

formed by the reaction of an A-block material selected from the groupconsisting of fatty acids and rosin acids and a B-block materialselected from telechelic diols.
 8. The container of claim 1 wherein thefirst polymeric material comprises a polyolefinic material selected frompolyethylene, polypropylene, polybutylene, and polystyrene.
 9. Thecontainer of claim 8 wherein the polyolefinic material comprises lowdensity polyethylene.
 10. The container of claim 1 wherein the secondpolymeric material is selected from ethylene vinyl alcohol copolymer,glycol modified polyethylene terephthalate, and polycarbonate.
 11. Thecontainer of claim 1 wherein the A--B--A resin is a compound of theformula ##STR2## wherein each is selected from the group consisting ofalkyl, aryl, alkylaryl, acyl and arylacyl derivatives of an aliphatic oraliphatic/aromatic mono-acid with a molecular weight of from about 200to about 500 daltons, each of R² and R³ is selected from the groupconsisting of H, CH₃ and C₂ H₅ ; each of D and D¹ is selected from thegroup consisting of O, --NR³ --, S and CO₂ ; each of n and p is andinteger from 0 to 8, provided when p=0, n is greater than 0; each of mand r is an integer selected from 0 to 20, provided when r=0, m isgreater than 0; and s is an integer from 2 to
 20. 12. The container ofclaim 11 wherein D and D¹ are CO₂ ; R² is hydrogen; n is 5; m is 1, r is0; and s is an integer from 5 to
 12. 13. The container of claim 12wherein s is
 8. 14. The container of claim 13 wherein R is a rosin acidderivative.
 15. The container of claim 11 wherein D and D¹ are CO₂ ; R²is hydrogen; m, n and r are 1; p is 3 and s is an integer from 5 to 12.16. The container of claim 15 wherein s is
 8. 17. The container of claim15 wherein R is a rosin acid derivative.
 18. The container of claim 11wherein D and D¹ are CO₂ ; R² is hydrogen; m, n, p and r are 1; and s isan integer from 5 to
 12. 19. The container of claim 18 wherein s is 8.20. The container of claim 18 wherein R is a rosin acid derivative.